REVIEW ARTICLE Illustrated Review of the Embryology and Development of the Facial Region, Part 2: Late Development of the Fetal Face and Changes in the Face from the Newborn to Adulthood P.M. Som and T.P. Naidich ABSTRACT SUMMARY: The later embryogenesis of the fetal face and the alteration in the facial structure from birth to adulthood have been reviewed. Part 3 of the review will address the molecular mechanisms that are responsible for the changes described in parts 1 and 2. art 1 of this 3-part review primarily dealt with the early em- first make contact, each is completely covered by a homoge- Pbryologic development of the face and nasal cavity. Part 2 will neous epithelium. A special epithelium arises at the edge of discuss the later embryonic and fetal development of the face, and each palatal shelf, facilitating the eventual fusion of these changes in facial appearance from neonate to adulthood will be shelves. The epithelium on the nasal cavity surface of the palate reviewed. will differentiate into columnar ciliated epithelium. The epi- thelium on the oral cavity side of the palate will differentiate Formation of the Palate into stratified squamous epithelium. Between the sixth and 12th weeks, the palate is formed from 3 The 2 palatal shelves also fuse with the triangular primary pal- primordia: a midline median palatine process and paired lateral ate anteromedially to form a y-shaped fusion line. The point of palatine processes (Fig 1). In the beginning of the sixth week, fusion of the secondary palatal shelves with the primary palate is merging of the paired medial nasal processes forms the intermax- marked in the adult by the incisive foramen. The fusion of the 2 illary process. From this, a wedge-shaped primary anterior mes- palatal shelves also results in a lengthening of the nasal cavity and enchymal mass extends posteriorly, between the internal surfaces carries the posterior choana back toward the pharynx. Ossifica- of the developing maxillae, to form the primary palate (Fig 1B). tion gradually occurs in the primary palate and then extends into The primary palate then gives rise to the premaxilla, the anterior the palatal processes to form the hard palate. The posterior por- median portion of the maxilla that encloses the 4 upper incisors. tions of the palatal processes do not ossify. Rather they extend The secondary palate is the primordium of the remaining hard posteriorly to the nasal septum and fuse to form the soft palate ad and the soft palates. Later in the sixth week, paired lateral palatine midline uvula.1,2 processes arise as medial mesenchymal projections from each maxillary process (Figs 1 and 2). Initially these grow inferiorly, Nasal Septum between the developing tongue and the developing alveolus (see The nasal septum develops downward and posteriorly from the the sections on the mandible and teeth) (Fig 2A). However, as the internally merged medial nasal processes and the frontonasal pro- maxilla and mandible elongate, the tongue is pulled downward cess. Fusion between the nasal septum and the palatal processes away from the lateral palatal processes. During the seventh and starts anteriorly during the ninth week and is completed by the eighth weeks (Fig 2B), the palatal shelves elevate into a hori- 12th week (Fig 2).1,2 zontal position above the tongue (Fig 2C,-D). This change in orientation is facilitated by the release of hyaluronic acid by the Mandible mesenchyme of each palatal process. When the palatal shelves During the fourth-to-fifth weeks, the mandibular processes gradually enlarge and merge in the midline. Between the fifth From the Department of Radiology, Mount Sinai School of Medicine, New York and eighth weeks, neural crest cells of the first pharyngeal arch University, New York, New York. give rise to left and right cartilaginous rods called the Meckel Please address correspondence to Peter M. Som, MD, Department of Radiology, cartilages. These form the cores around which the membra- The Mount Sinai Hospital, One Gustave Levy Place, New York, NY 10029; e-mail: [email protected] nous bone of the lower jaw develops. The mandibular pro- Indicates open access to non-subscribers at www.ajnr.org cesses form the lower lip, the lower jaw, and the lower cheek http://dx.doi.org/10.3174/ajnr.A3414 regions. The mentum marks the site where the 2 mandibular 10 Som Jan 2014 www.ajnr.org FIG 1. Drawings from below show the development of the palate from 6 to 7 weeks (A), 7 to 8 weeks (B), and 8 to 10 weeks (C). The lateral palatine processes grow medially and eventually merge in the midline and with the intermaxillary segment (primitive palate). The incisive canal marks the junction of the primitive and secondary palates. (Modified with permission from Levine HL, Clemente MP, eds. Chapter 1, Surgical Anatomy of the Paranasal Sinus. China: 2005. Sinus Surgery Endoscopic and Microscopic Approaches. Figures 1–3. Thieme Medical Publishers Inc., Georg Thieme Verlag Stuttgart). FIG 2. Serial frontal diagrams (A–D) from approximately 6–10 fetal weeks shown just posterior to the intermaxillary segment illustrate the progressive development of the secondary palate and its fusion with the nasal septum. processes merge in the midline. A partial or incomplete merger gingivae (Fig 3A). A small midline remnant of the labiogingival of these mandibular processes forms the common midline chin lamina persists as the frenulum of the upper lip (Fig 3B). dimple or cleft. Shortly after in the sixth week, a second lamina, the dental Gingiva, Lips, and Teeth lamina, arises in the more buccal margin of the developing Until the end of the sixth week, the primordial jaws comprise gingiva of both jaws. The dental lamina eventually will give rise only masses of mesenchymal tissue with no differentiation of to 10 spherical tooth buds that penetrate the mesenchyme of the lips and gingivae. At the end of the sixth week, a curvilinear both jaws. The tooth buds first appear in the anterior mandible thickening of ectoderm, the labiogingival lamina, grows into followed by the anterior maxilla. Budding then continues pro- the underlying mesenchyme. Most of this lamina degenerates, gressively posteriorly to form 10 deciduous teeth each in the creating a labiogingival groove or sulcus between the lips and maxilla and mandible. From about the 10th fetal month, deep AJNR Am J Neuroradiol 35:10–18 Jan 2014 www.ajnr.org 11 FIG 3. Drawing from above and in front (A) of the developing lips and gums. The labiogingival lamina develops in the common mesenchymal tissue of this region. When it dissolves, the labiogingival sulcus that remains separates the lips and gums. The dental lamina develops just behind this region and will give rise to the dental buds, which will form the deciduous and permanent teeth. B, Frontal photograph shows the frenulum of the upper lip, the only remaining vestige of the labiogingival lamina. sors and arise directly from posterior extensions of the dental laminae. By the 10th week, a mesenchymal condensation called the “dental papilla” invaginates into each tooth bud, resulting in the formation of a cup-shaped enamel organ (Fig 5A). The enamel organ has an inner epithelium, an outer epithelium, and a middle enamel reticulum (enamel pulp) (Fig 5B,-E). The dental papilla contains attenuated collections of stellate cells, which eventually give rise to most of the tooth proper, includ- ing the pulp cavity, the dentin, and the vasculature of the tooth. The enamel organ and dental papilla are surrounded by a mes- enchymal concentration called the “dental sac.” This sac will develop into the fibrous connective tissue (periodontal liga- ment) that attaches the roots of the teeth to the alveolar bone (Fig 5C). The inner enamel epithelium later differentiates into the ameloblasts that produce the tooth enamel. By the third month, the mesenchymal cells in the dental papilla adjacent to the inner enamel epithelium differentiate into odontoblasts. FIG 4. Lateral drawing of the developing teeth. Note that the perma- These produce predentin and deposit the predentin adjacent to nent teeth (blue) develop medial to the deciduous teeth. (Modified the inner enamel epithelium (Fig 5C). In the sixth month, the with permission from Frank H. Netter, Atlas of Human Anatomy, 5th Edition, Saunders Elsevier, Philadelphia, 2011, Figure 56. Netter predentin calcifies to become the dentin of the tooth. Illustrations from www.netterimages.com, ©Elsevier Inc, All rights The crown of the tooth refers to that part of the tooth covered reserved). by enamel and projecting above the gum line. The formation of the tooth root begins when the inner and outer enamel layers components of the dental lamina create the buds for the per- penetrate into the mesenchyme to form the epithelial root sheath manent teeth along the lingual aspects of the deciduous teeth (Hertwig epithelial root sheath) (Fig 5D). Root formation takes (Fig 4). The permanent molar teeth have no deciduous precur- place after the crown formation has been completed. The root 12 Som Jan 2014 www.ajnr.org FIG 5. Drawings of the progressive development of the teeth from the tooth bud stage that comes from the dental lamina to the adult tooth. (Modified from http://embryology.ch/anglais/sdigestive/gesicht05.htm and www.embryo.chronolab.com/teeth.html). canals arise through extension and later fusion of the enveloping and eighth weeks, sheet-like collections of premyoblasts and early root sheath (Fig 5F). The inner cells of the dental sac differentiate myoblasts extend from this attenuated mesenchyme to form 5 into cementoblasts, which will produce the cementum of the laminae on each side of the face, which extend into the superficial tooth. Increasing amounts of dentin reduce the pulp cavity to the portions of the future temporal, occipital, cervical, and mandib- narrow root canal through which the vessel and nerves pass.